Electron beam (E-beam) lithography systems utilizing a variably shaped spot (VSS) without high speed focus correction have operating limitations; i.e., lower thruput or less distinct spot images. The electron beam is shaped to various sizes, having spot areas varying typically in the range of 1-100 .mu.m.sup.2 and consequently beam focus and therefore current densities vary. Additionaly, such E-beam systems require high speed beam switching times in order to provide system throughputs at acceptably high values. Typical switching speeds must be in the order of 25 ns. Existing deflection control circuits, such as high analog voltage parallel plate deflection control devices cannot meet this timing requirement.
Prior art representative of such systems is typified in IBM TDB vol. 21, No. 8, page 3185 and U.S. Pat. Nos. 3,192,433, 3,903,455 and 4,132,908.
Typically in such prior art systems, a single deflection coil is driven by an output generated as a consequence of the sum of a number of D/A current drivers. Each of these drivers has some device and parasitic capacitances. When the drivers are summed at a single coil, the capacitances add and tend to limit switching speed.
Also, in driving such coils at high speeds, high switching voltages are required since V=Ldi/dt. Thus, high voltages and high currents require large switching devices. Such switching devices in turn have large capacitances which tend to resonate with coil inductance. This also tends to limit switching speed.